Power converters typically are required by imposed safety requirements to provide full DC galvanic isolation between the input source common return circuit and the output load common return circuit. This full DC galvanic isolation is normally provided by use of a power transformer which blocks DC current flow between the primary and secondary circuitry (e.g., in galvanic isolation there is no DC current path across the isolation barrier). Power converters typically also require the output signals to be regulated. That is, the output voltage and/or current must somehow be sensed and made to fall within some boundary limit of specified values. To achieve this, without breaching the DC galvanic isolation provided by the power transformer, commonly requires a secondary to primary feedback path including an opto-isolator, signal transformer or some other isolating signal means, to provide the desired galvanic isolation between input and output.
The regulating circuitry is normally divided into circuitry at both the primary and secondary sides of the converter. The secondary side circuitry senses the output signal to be regulated and the primary side circuitry controls a power switch to achieve the desired regulation. The division of the regulating circuitry between input and output and the galvanic isolating devices required in the feedback path increases the parts count of the converter and increases its size and expense. It is desirable to reduce the parts count to enhance circuit reliability. It is also desirable to reduce both cost and circuit size in many applications. Among existing methods that eliminate the need for a separate isolating feedback path are the techniques of implicit primary side sensing and post secondary regulation.
Post secondary regulation, implies an added power processing module subsequent to the power converter itself. It adds complexity to the overall system and significantly increases the system parts count. It additionally detracts from the power system efficiency since it uses a separate power processing module in tandem with the primary power converter and the overall efficiency is the product of the two individual efficiencies.
Implicit sensing on the primary side of the converter need not significantly impact the efficiency of the power processing system and it may be implemented within integrated circuitry that combines it with other control functions of the converters. One well known method of implementing implicit sensing is to add a tertiary winding to the power transformer to enable sensing of a signal related to the output signal of the converter. A typical implicit sensing arrangement in a flyback type power converter senses a voltage from a tertiary winding on the power transformer. This voltage is sensed without compromising the DC galvanic isolation between input and output. However, the regulation based on this sensed voltage tends to be inaccurate since the tertiary winding voltage is not necessarily directly proportional to the output load voltage. Estimated voltages derived from this tertiary winding are inaccurate since estimated currents tend to be a constant while the actual output load currents vary.
Implicit sensing of output voltages and currents is typically dependent on static estimating circuit arrangements that do not accurately reflect the dynamic nature of the power converters output signals. Such arrangements tend to be relatively inaccurate in estimating the output signals at the primary side. Neither output currents nor output voltages are reflected accurately in these arrangements. Regulation accuracy using these implicit sensing schemes is typically 10% to 20% and is functional only over a very limited range of load current. To be effective and achieve accurate regulation, an implicit sensing scheme at the input must accurately reproduce the signal conditions at the output of the converter.